1. Field of the Invention
The present invention relates to the structure of electronic circuitry cabinets and, more particularly, to the arrangement of a plurality of interchangeable circuit board modules in a circuitry cabinet and to the positive prevention of erroneous insertion of a circuit board module through use of mechanically cooperating code elements.
2. Description of the Prior Art
The widespread use of printed circuit boards in conjunction with the miniaturization of electronic circuitry components has made it possible to arrange the majority of electronic control components directly on the surface of the printed circuit boards and, in the case of extensive circuitry requirements, to arrange a number of interchangeable printed circuit board modules within a circuitry cabinet from which each module can be readily withdrawn for servicing access or replacement.
Interchangeable circuit board modules are normally so arranged that the insertion of a module automatically establishes multi-conductor contact between it and the other circuit board modules through a data bus, for example, or between the circuit board module and a multi-pin plug connector at the end of a cable.
An electronic circuitry cabinet may contain a considerable number of outwardly identical circuit board modules which are supported in parallel insertion guides along which each module can be individually inserted and retracted. Obviously, the insertion of a circuit board module into a module slot other than the one for which it was designed will not only lead to malfunction of the control center, but, more often than not, will cause damage to some of the electronic control components on the module. The damage may be of the kind which is not immediately evident.
The desire to preclude the possibility of an electronic control center being powered-on with a circuit board module inserted in the wrong slot of its circuitry cabinet has already led to various suggestions aimed at signaling the insertion error and/or preventing powering-on, or at physically preventing the insertion--at least the complete insertion--of a circuit board module.
One prior art solution is disclosed in the German Offenlegungsschrift (Published Application) No. 31 00 393 which suggests the attachment of an insertion verification clip to the outer edge of each of a bank of circuit board modules in such a way that the verification clips are vertically staggered in a regular pattern and the clip of each module establishes electrical contact with the higher clip of an adjacent module on one side and the lower clip of an adjacent module on the other side. This arrangement provides an identification of each module through the vertical location of its clip. The chain of electrical contacts can be used to produce a warning signal and/or to prevent powering-on.
A major shortcoming of this arrangement lies in the fact that the higher one of two adjoining clips blocks the retraction movement of the lower one, with the result that the circuit board modules can only be inserted and removed in a predetermined sequence.
Another solution is disclosed in the German Gebrauchsmuster (Utility Patent) No. 75 32 854. This publication suggests the arrangement of insertion-preventing code elements on each circuit board module and on its associated stationary contact bar in the circuitry cabinet. The code elements take the form of comb-shaped members with teeth and tooth gaps engaging each other. The distinct code patterns on the various circuit board modules are produced by snap-inserting one or more code clips into selected tooth gaps of initially identical code members which are attached to the contact edge of the circuit board modules, and by breaking off one or more teeth of initially identical stationary code members in the module segments of the circuitry cabinet in places which correspond to the places where the circuit board modules carry inserted code clips.
Among the disadvantages of this solution are its complexity in terms of parts and assembly skills required and the risk of accidental destruction of the code pattern through breakage of a stationary tooth, when an attempt is made to insert a circuit board module into the wrong slot in the circuitry cabinet. The production of the distinct code patterns requires the manual modification of initially identical code members in the course of the assembly operation or in the field, when a circuit board module has to be replaced.